The three were among six individuals – including one woman – whom investigators arrested on Thursday. The group is understood to have included a former police and community support officer for the Metropolitan police, but Scotland Yard would not confirm that detail.

The raids were carried out by counter-terrorism officers investigating a suspected plot to attack targets in Britain. Although police said they were not linked to any threat to the Olympics, officials say they will intervene quicker than usual in disrupting suspected plots in the runup to the event, which counter-terrorism forces fear is a major target.

One person was shot with a Taser stun gun and Scotland Yard said armed officers had been deployed during the arrests, which is an unusual decision and indicative of the risk counter-terrorism chiefs thought officers faced in detaining the suspects.

Five men and one woman were arrested in what police said was an intelligence-led operation believed to have gone on for several weeks.

The six were arrested on suspicion of the commission, preparation or instigation of acts of terrorism. Officials believe they are dealing with a plot inspired by al-Qaida ideology.

The six people were taken to a south-east London police station for questioning. In another unusual occurrence, a 29-year-old man was arrested in a west London street. Usually, police prefer to arrest suspects at their homes.

A Met spokesman said those arrested included a 21-year-old man and a 30-year-old woman who were detained at separate residential addresses in west London. Three other men, aged 18, 24, and 26, were arrested at residential properties in east London.

One of those held was a white British convert to Islam who grew up in Dorset, a source in the Muslim community said. Police would not confirm the claim.

A Met spokesman said: "Searches under the Terrorism Act 2000 are being carried out at eight residential premises in east, west and north London and one business premises in east London. Public safety remains our overriding concern. Today's operation in London is not linked to the Olympic and Paralympic Games."

It is understood MI5 played an active role in the operation. Investigative sources suggested there was no imminent threat but officers believed there was enough evidence to act.

MI5 and the police have said repeatedly they were monitoring a number of groups that could pose a threat to national security, and they had been focusing on "nipping plots in the bud".

Scotland Yard and MI5 are on a state of high alert, with less than a month to go before the Games. Police may make more arrests in the coming weeks as officers become more risk-averse.